Treatment of nonferrous alloy castings



1936 w. c. DEVEREUX ET AL 2,059,984

TREATMENT OF NONFERROUS ALLOY CASTINGS Filed Dec. 12, 1934 \l 1 I a): A I I I 7eir yorflep 2,059,984 PATENT QFFlCE murmur or nomaaousaimr cas'rmos Wallace Charles Devereux, Slough, and Christopher Sibley Oliver, Coventry, England, assignors, by mesne assignments, to The Dow Chemical Company, Midland, Mich a corporation of Michigan Application December 12, 1934, Serial Nd. 757,204 In Great Britain June 18, 1934 10 Claims.

This invention relates to the treatment of nonferrous alloy castings with more especial reference to the treatment of castings composed of magnesium alloys which castings, in spite of the most rigorous control and attention during the casting operation are inclined to exhibit some permeability under the rigorous tests applied to articles of this character and in the manufacture of which accordingly there has hitherto been a large number of rejections.

The present invention has for its object to provide an inexpensive process for the treatment of alloy castings which effectively removes permeability without necessarily modifying the surface a appearance or affecting the inherent structure of the casting and by which the number of rejected castings may be substantially reduced or even eliminated. h

In accordance with the present invention alloy castings are impregnated with a readily fusible alloy having a low viscosity when in fluid con-' dition so as to be capable of penetrating to the inter-crystalline crevices in the structure of the casting and adapted on solidification to unite with the parent alloy and thereby render the structure impermeable under pressure.

In selecting the fusible alloy for the impregnating treatment it is desirable, if not necessary, to have regard to the contraction characteristics on cooling, since if the coefllcient of contraction of the fusible alloy is such that on solidification and cooling it contracts to' a greater extent than the parent casting, voids are likely to occur in the casting, and the object of the process will be defeated. I

Further, it is desirable to employ as the impregnating alloy some eutectic mixture which will further alloy or amalgamate with the parent casting on solidification and subsequently in such manner that by the amalgamation the characteristics of the inserted material are altered, for instance by a material elevation of the melting point, thereby to guarantee effective union under working condition.

To this end the constitution of the impregnating alloy may vary according to the constitution of the alloy from which the casting to be treated is manufactured, and in general alloys will be utilizedwhich are fusible at temperatures below 100 C., whereby impregnation can be effected without recourse to unduly high temperatures and without likelihood of distorting the castings.

As aforementioned, castings of magnesium alloys present a greater inclination to weep than do castings .of aluminium alloys or other kindred materials, and for the impregnation of such magnesium alloy castings an alloy composed of tin, lead, bismuth and cadmium having a melting point of approximately 88.05 C. has been found especially suitable.

In the practice of the invention, an impregnat ing alloy of sufiiciently low melting point is obtained by employing a fusible alloy containing bismuth, lead and tin, while if a melting point below C. is required a cadmium component may be added. Woods metal is suitable for the purpose, although, preferably, the bismuth content of the fusible alloy used for impregnation, while not being less than 40%, is present in substantially this proportion and consequently is a lower proportion than in Woods metal.

Applying the process according to the invention to a magnesium alloy casting, the casting is heated to and maintained at a temperature in the vicinity of 160 C. in any appropriate'manner during the treatment, and the impregnating alloy, previously heated to a temperature of C., is run into or on to the casting and is forced therethrough by the application of suflicient pressure. It has been found that excessive pressure is not called for owing to the fluidity and penetrating powerof the molten-alloy which, under the conditions of treatment, impregnates the casting and weeps freely from the opposite surface thereof.

The surface adhesion between the impregnating alloy and the parent structure is, however, negligible, and, at the conclusion of the treatmerit, the surface "of the casting does not exhibit any .-trace of the impregnation'to which it has been subjected.

On the other hand, castings which exhibit a pronounced tendency to weep under pressure prior to impregnation-prove elfectively impermeable after the treatment even under temperature conditionsi considerably above the melting point of the fusible alloy which would seem to show that on solidification the fusible alloy unites with and forms an integral part of the metallic structure of the casting.

The invention will be further described with reference to the accompanying explanatory drawing which illustrates by way of example one form of apparatus for carrying the process into efiect, and in which the numeral I generallydesignates the article composed ofma'gnesium alloy which it is desired to render impermeable, in the case illustrated, the rear crank-case cover of a radial aircraft engine. v

In the first place, any apertures, there being five in the casting illustrated, are effectively blocked by means of removable clamp plates or solid washers 2 faced with appropriate heat resistant packing material 3 such as Hallite.

To the most convenient aperture, the lower end of a plunger or ram cylinder 4 is secured, either with. or without" packing material, by means of clamping rods 5 passing over the base 6 of such cylinder 4 around the casting I, and being securely attached to a pedestal or machine bed I.

Theassemblyincludingthecasting l,plunger cylinder 4 and bed I are then heated inan electric furnace or in other appropriate manner to a temperature of about 160 C., and meanwhile the impre nating alloy is melted.

After the casting has been heated sufiiciently to attain a temperature of 160C. throughout, the assembly is withdrawn from the furnace and placedinapressinsuchmannerastoleave its major surface area exposed, whereafter the fused alloy is poured through the ram cylinder until the interior of the casting is filled.

Thereupon the. ram or plunger, designated 8, n replaced in its cylinder and the press closed to appiypressurethroughittotbebodyof liquid alloy located in the casting.

Under light pressures the fusible alloy impregnatesthecastingandweepsfreelyfromitsouter face as a fine and substantially invisible mist or spray which, as it is cooled by the atmosphere, sublimesorsolidifiesonthebedas asoftmetallic wool.

The process is continued for a period depending on the form and shape of the casting-usually for about five minutesadditional quantities of fusible alloybeinginsertedinthecylinderl, if required, and while the casting is still above the melting temperature of the fusible alloy the pouredfrom theinteriorofthecastinglinto an appropriate container for subsequent use in thenexttreatment.

'lhecastingisthenpermittedtocoohandduriii cadmium in the proportions of 42%, 24.6%, 23% and 10.8% respectively on castings of magnesium alloyed with aluminum. copper, sine, cadmium and/or. manganese.

The particular apparatus illustrated is especially suited for applying the treatment to hollow castings such as pump casings and crank cases, e. g. for aero engines, but for other articles recoursemaybehadtoaheatedvat ortankwith which is associated evacuating mechanism, the casting being placed in the vat or tankwhich is then evacuated and filled with fusible alloy under pressure, thereby to effect impregnation. the pressure being subsequently released and the impregnatedcasting removed and permitted to cool.

By the treatment according to the present invention castings are rendered substantially impermeable and consequently the number of re- Jected castings is substantially reduced.

What we claim is:-

1. A process for the treatment of magnesium alloy castings to render them impermeable, consisting in heating the castings to a temperature of about 160 C. throughout, forcing thereinto a fused alloy of bismuth, lead, tin and cadmium, whilemaintaining the temperature of the casting above the melting point of the fused alloy. and allowing the casting to cool and to amalgamate with the fused alloy on solidification.

2. In a process for the treatment of porous non-ferrous alloy castings comprising impregnating the castings with an alloy having a low melting point and a low viscosity when in fluid condition, the steps which consist in heating the a casting throughout to a temperature considerably above the melting point of the impregnating alloy and'maintaining it above such melting temperature while forcing the impregnating alloythmughthematerialofthecastingunder pruaure.

3.Aprocessaccordingtoclaim2inwhich the impregnating alloy employed is composed substantiallyofbismuthleadandtimsuchallcy being fusible at temperatures below 100' C.

4. Aprocessaccordingtoelaim2inwhichtbe impregnating alloy employed is composed substantiallycfblsmuthleadtinandeadmium, such" alloy being fusible at temperatures below 5. A process for impregnating porous magnuium alloy castings to render the same impermeable which comprises forcing under pressure a molten alloy of low melting point and low viscosity when in fluid condition into the voids inthematerialofsucheasting,whilemaintainingthetemperatureofthecastingabovethe meltingpointofthefusedalloy,andallowlngthe treated casting to cool.

6.Aproeessaccordingtoclaim5inwhich the impregnating alloy employed is composed substantially of bismuth, lead and tin, such allow being fusible below 100' C.

7. Aprocessaccordingtoclaimfainwhichthe impregnating alloy employed is composed substantially of bismuth, lead, tin and cadmium, such alloy being fusible below 75 C.

8. A process according to claim 5 in which the impregnating alloy employed is composed of approximately 42 per cent bismuth, 24.5 per cent lead, 23 per cent tin. and 10.5 per cent cadmium.

9. Non-ferrous alloy castings rendered impermeable by treatment according to the process defined in claim 2.

l0. Magnesium alloy castings rendered impermeable by treatment according to the process defined in claim 5.

WALLACE CHARLES DEVEREUX. CHRISTOPHER SIBLEY OLIVER. 

